"Tonight has brought a conclusion to a case that illustrates how methamphetamine can have such a monstrous impact on so many lives," Nixon said in a statement.

Middleton was convicted of killing Randy "Happy" Hamilton, Stacey Hodge and Alfred Pinegar in 1995 out of concern that they would tell police about his methamphetamine dealing. Middleton's girlfriend, Maggie Hodges, is serving life in prison after pleading guilty to second-degree murder in all three cases.

Middleton's attorneys contend that the wrong man was arrested, citing new evidence that included a witness who came forward in February.

"We're looking at a situation where if (Middleton) had zealous representation at trial, he likely would have been acquitted," attorney Joseph Perkovich said.

Concerns about whether Middleton was mentally fit for execution, though, spurred a long delay in the execution, which occurred 19 hours after it was originally scheduled, at 12:01 a.m.

A federal judge granted a stay of execution late Tuesday, citing a need for a hearing to determine if Middleton was mentally ill. Courts have held it is unconstitutional to execute the mentally ill.

A federal appeals court overturned the stay later Wednesday and neither the U.S. Supreme Court nor the Missouri Supreme Court would halt the execution.

Missouri has executed one man each month since November, with the exception of May, when the U.S. Supreme Court halted the execution of Russell Bucklew. Bucklew suffers from a rare congenital condition that causes weakened and malformed blood vessels as well as tumors in his nose and throat. The 8th U.S. Circuit Court of Appeals plans a hearing on Sept. 9 to determine if lethal injection could cause him to suffer because of his medical condition.